A typical dispensing device for supplying a liquid, such as hot melt adhesive, generally includes a dispenser body having a valve stem that opens and closes a dispensing orifice. The valve stem is usually operated by pressurized air to dispense discrete amounts of pressurized liquid. One or more liquid seals within the device prevent the migration of liquid between the liquid and air passages of the device.
Devices generally related to the present invention include a liquid passage adjacent the dispensing orifice and an air passage or chamber at an opposite end of the device. The air passage contains a piston connected to the valve stem on one side and may include a spring-return mechanism on the other side. Under sufficient air pressure, the piston and valve stem may be moved in a direction away from a valve seat to discharge liquid. When the air pressure on one side of the piston is relieved, the spring-return mechanism will automatically return the valve stem to a normally closed position against the valve seat. Air pressure may also, or alternately, be used to close the valve stem. The spring-return mechanism may be used to adjust the valve stroke such as by varying its compression, thereby varying the amount of air pressure required to open the valve. Adjustment of the spring compression will also adjust the biasing force used to close the valve.
Dispensing devices related to the present invention generally situate at least one dynamic seal between the dispenser body and the valve stem to prevent liquid from leaking out of the liquid passage and into the air passage. Dynamic seals are conventionally understood to be seals between two surfaces that move relative to one another. For example, many dispensing devices use one or more lip seals having a coil spring that supplies a radially-directed inward force to bias an annular lip against the valve element. The annular lip generally includes a bearing edge that provides a wiping action as the valve stem moves relative to the seal. In other dispensing devices, a seal, such as a standard O-ring or spring-energized cup seal, tightly fit around the valve stem for axial movement therewith along an inner surface of the dispenser body. In either case, the relative motion between the valve stem and the dispenser body causes the seal to wear and therefore lose its ability to seal properly.
Consequently, to prevent liquid from migrating into the air passage and causing major damage to the dispensing device, the seals are generally replaced as part of a routine, in-house maintenance program. To perform the maintenance, the production line is temporarily shut down so that the dispensing device may be disassembled and the seals replaced. Shutting down the production line, however, increases costs due to lost production and lost time. Thus, it is desirable to minimize the time it takes to replace the seals. Current dispensing devices, however, have complex seal designs that include many separate, individual parts that must be disassembled in the correct manner, then reassembled in the correct manner. As a result, seal replacement in current dispensing devices can be a tedious and time consuming process that increases production line down time and increases costs.
Accordingly, there is a need for an improved device for dispensing viscous liquids, such as hot melt adhesives, which has an improved seal assembly that can be replaced in a quick and convenient manner thereby reducing down time and overall costs.